Question: Simplify the following expression and state the conditions under which the simplification is valid. You can assume that $q \neq 0$. $y = \dfrac{5q + 50}{-q^3 - 2q^2 + 80q} \times \dfrac{q^3 + q^2 - 6q}{q + 3} $
Answer: First factor out any common factors. $y = \dfrac{5(q + 10)}{-q(q^2 + 2q - 80)} \times \dfrac{q(q^2 + q - 6)}{q + 3} $ Then factor the quadratic expressions. $y = \dfrac {5(q + 10)} {-q(q + 10)(q - 8)} \times \dfrac {q(q + 3)(q - 2)} {q + 3} $ Then multiply the two numerators and multiply the two denominators. $y = \dfrac {5(q + 10) \times q(q + 3)(q - 2) } { -q(q + 10)(q - 8) \times (q + 3)} $ $y = \dfrac {5q(q + 3)(q - 2)(q + 10)} {-q(q + 10)(q - 8)(q + 3)} $ Notice that $(q + 10)$ and $(q + 3)$ appear in both the numerator and denominator so we can cancel them. $y = \dfrac {5q(q + 3)(q - 2)\cancel{(q + 10)}} {-q\cancel{(q + 10)}(q - 8)(q + 3)} $ We are dividing by $q + 10$ , so $q + 10 \neq 0$ Therefore, $q \neq -10$ $y = \dfrac {5q\cancel{(q + 3)}(q - 2)\cancel{(q + 10)}} {-q\cancel{(q + 10)}(q - 8)\cancel{(q + 3)}} $ We are dividing by $q + 3$ , so $q + 3 \neq 0$ Therefore, $q \neq -3$ $y = \dfrac {5q(q - 2)} {-q(q - 8)} $ $ y = \dfrac{-5(q - 2)}{q - 8}; q \neq -10; q \neq -3 $